In some public land mobile networks (PLMNs), such as in a Global System for Mobile (GSM) communications network, calls are routed to mobile phones operating in the network using home location registers (HLRs). The HLRs store data about where in the network a particular mobile phone is operating. When a mobile phone roams to a new radio access network (RAN) node, the phone connects to a mobile switching center (MSC) associated with that RAN node. The MSC identifies the phone and informs an HLR associated with the phone that the phone is now currently in wireless communication with the MSC. If an incoming call directed to the phone is then received in the network, the network first contacts the HLR associated with the phone to learn the identity of the MSC that is currently managing wireless communication with the phone. The incoming call is then routed through the MSC to the phone.
During periods of high network usage, call congestion may require an MSC network to drop (i.e., terminate) some existing calls and/or prevent some new calls from being completed. However, some network operators now offer their subscribers differentiated services in the form of priority services that can enable a particular subscriber to receive priority processing of calls. For example, a subscriber that receives priority services may be guaranteed a particular level of quality of service (QoS) for all calls made from the subscriber's phone. Also, a subscriber that receives priority services may be guaranteed that, during periods of high network usage, non-priority service calls will be dropped before priority service calls. For example, priority services include wireless priority service (WPS), enhanced multi-level precedence and pre-emption service (eMLPP), and circuit switched allocation/retention priority (CARP) service. Priority services can be provisioned using supplementary service (SS) codes, such as defined in the Third Generation Partnership Project (3GPP) technical specification (TS) 29.002.
A priority service status of a subscriber is generally known only within the subscriber's serving MSC. Therefore, for inter-MSC calls, an MSC associated with a caller device may not know the priority service status of a callee device; and, similarly, an MSC associated with a callee device may not know the priority service status of a caller device.
Transcoder free operation (TrFO) concerns the transport of compressed speech, from legacy mobile phones, in a packet transport network through elimination of unnecessary coding and decoding of a call signal by intermediate elements in a bearer path. TrFO uses out of band signaling to determine a negotiated coder-decoder (codec) type to be used at a caller device and a callee device. If the two devices can use compatible codec operations, it may be possible to traverse an entire packet network using only compressed speech from a preferred codec.
However, in an inter-MSC call, when a high-priority services subscriber calls a low-priority services subscriber using TrFO, codec negotiation may result in the use of a codec associated with the low-priority services subscriber. That can be undesirable, as it prevents the network operator for the high-priority services subscriber from being able to guarantee a particular priority service status for inter-MSC calls placed either from or to the high-priority services subscriber.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.